With the advent of heart-rate (HR) monitors in 1978 by Polar Electric Oy, the Finnish company now known simply as Polar, endurance athletes have been able to monitor their workout intensity accurately and inexpensively.
Today, numerous companies manufacturer affordable heart-rate monitors which not only display current HR but also log exercise time, interval duration, ... Top of the line models include GPS navigational systems and computer interface capabilities.
While they aren't a necessity for monitoring your HR, you can still use a clock and a finger to take your pulse, they are convenient and easy to use. The American College of Sports Medicine (ACSM) recommends using heart rate as an indicator of exercise intensity when training to obtain minimal cardiovascular fitness.
The ACSM cardiovascular guidelines include: exercising at an intensity of 55/65 - 90 percent of maximal heart rate (HRmax), 40/50 - 85 percent of heart rate reserve (using the Karvonen formula) or a rating or perceived exertion (RPE) of 3 to 7; exercise 3 to 5 days or more per week; for a duration of 20 to 60 minutes.
One important point for you to consider, however, is that heart rate can only be used when performing aerobic exercise. Heart rate is a poor, and inaccurate, indicator or anaerobic exercise intensity or workload.
A better gauge of anaerobic intensity is power output, repetition maximum (used with weight training), or pace & duration used with interval training.